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Mercury's tenuous atmosphere (exosphere) contains hydrogen, helium, and oxygen, as well as smaller amounts of sodium, potassium, calcium, and magnesium. This graphic shows sodium concentration near Mercury as detected by the MESSENGER spacecraft in October 2008.
Click on image for full sizeImage courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

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Atmosphere of Mercury

Mercury has very little atmosphere. The planet's small size means that its gravity is too weak to hold down a normal atmosphere. There is a very thin atmosphere around the planet. Mercury's thin atmosphere is constantly being "blown away" into space by the pressure of sunlight and by the solar wind. The tiny planet's atmosphere is also constantly replenished.

Mercury's atmosphere contains small amounts of hydrogen, helium, and oxygen. It also has even tinier quantities of sodium, potassium, calcium, and magnesium. Hydrogen and helium come from the solar wind. Helium, sodium, and potassium are produced by radioactive decay in Mercury's crust. Micrometeorites vaporize surface rocks, adding gaseous sodium, potassium, and calcium to the thin, exotic "air". All of these gases are soon ionized by heat, sunlight, or radiation, and are quickly carried away from Mercury by the solar wind and interactions with Mercury's magnetic field. Atmospheric pressure at the planet's surface is less than one trillionth of Earth's (around one nanopascal or 10-14bar).

Temperatures at the surface range between 100 and 700 kelvins (-280° F to 800° F or -173° C to 427° C). Lead melts at 600 kelvins! This large range in surface temperature is possible because
Mercury is so close to the Sun (a year is only 88 Earth days long) and does
not have sufficient atmosphere present to moderate the range in surface
temperature.

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